Realization of Airy pattern acoustic bullets by depth of focus synthesis.

Energy concentration is a key parameter for the characterization of light or acoustic bullets. To the best of our knowledge, the highest encircled-energy ratio in the mainlobe reported is merely 25% for the solitary bullet waves and 30% for the nonsolitary bullet waves. This letter reports experimentally and theoretically on a new family of acoustic bullets, namely, Airy pattern ultrasonic bullets, realized by the depth of focus synthesis. The Airy pattern ultrasonic bullets, regarded as a beam with an ultra-long depth of focus, exhibit Airy pattern energy distribution and contain 83.8% of total energy in their mainlobe. In the depth of focus synthesis proposed, an ultra-long depth of focus could be longitudinally synthesized by successive acoustic foci by using an acoustic reflection cavity (ARC). In contrast to the well-known aperture synthesis which utilizes a platform motion to extend the aperture, the depth of focus synthesis employs multiple reflections in ARC to extend its physical dimensions. The Airy pattern ultrasonic bullets presented would be potentially useful in long-range ultrasound imaging, ultrasonic therapy, and acoustic wireless energy transmission.